FYI, in case anyone is curious, here are two examples, the current US coastal 
time zones. You can see that you need to get about 7 to 10 bits into the low 
order word to represent hours exactly.

FFFFBCF1_DCC00000      // -1499230800 = -18000 seconds = -5 hours
FFFF94B6_2E000000      // -8 hours or -28800 seconds

Charles


-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf 
Of Steve Smith
Sent: Friday, February 23, 2018 9:03 AM
To: IBM-MAIN@LISTSERV.UA.EDU
Subject: Re: Two summer time change questions

Timer units just happen to be what the high-order word of the traditional TOD 
clock counts.  Whether that figured as a design point back in the day I have no 
idea.  I presume the span covered (1900-2047) and the precision (specified as 
microseconds in bit 51, implying much room for more in the last 12 bits) were 
more important considerations.  Just my guess, though.

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